The present invention relates to a process of making highly dimensionally stable composite products from lignocellulosic material without the addition of synthetic resin binders and products produced therefrom where the final product is similar to conventional high density products made with high resin content.
The technologies of manufacturing wood-based composite products have not changed significantly since their original inception about 80 years ago (Masonite wet process for manufacturing thin hardboard). Essentially, the processes involve reducing wood into fibres, particles, chips, strands etc, adding synthetic resins and then consolidating them under heat and pressure to produce a composite product. Their physical properties, and therefore their end applications, are determined in large part, by the quantity and nature of the synthetic resins used to bind them. Urea- and phenol-formaldehyde are the most common resin binders in use. UF resin yields wood composite products for interior use, while PF, which is more expensive, is usually used in composite products intended for exterior use. Incremental improvements have been made by modifications to resins, methods of application, methods of producing feedstock, chemical additives for modification of feedstock, orientation of feedstock and pressing methods. However, the use of synthetic resins derived from petrochemicals remains as the main method of bonding.
One notable exception to the conventional processes of manufacturing is the Masonite process for thin hardboard, which differs from the conventional dry processes in that lignin, a component of wood, is used as a binder. No additional synthetic resins are added. However, the Masonite process is “wet,” in that significant quantities of water are required to wash out water solubles, which interfere with the bonding process, from the fibre feedstock. The washing process also results in about 30% loss of raw material. Furthermore, the resulting product is limited in thickness to less than 6 mm and possesses screen marks on the backside. For these reasons, only a few wet process hardboard mills remain in operation today around the world.
One major drawback of conventional lignocellulosic composite products is their dimensional stability, measured by thickness swelling and linear expansion. Wood is hygroscopic in nature. It will absorb moisture and in a humid environment, it will swell. Conversely, wood will lose moisture and shrink in a dry environment. The fluctuation of humidity around the wood results in dimensional changes in accordance with the changes of the surrounding humidity while direct contact with water causes great dimensional changes. This dimensional change is undesirable, particularly in the case of lignocellulosic composite products, such as particleboard, fibreboard, oriented strand board and high pressure laminate, because these composite products are compressed into a higher density than their original form in order to develop interfacial adhesive bonding. Dimensional changes not only weaken the glue bond holding the products together, but also result in physical changes which compromise the integrity of the application for which the product is used; i.e. warping, cupping, buckling, bowing, splitting and cracking.
Significant improvement of the dimensional stability of composite products produced by conventional methods is very expensive, requiring additional quantities of resin, longer pressing times, higher temperature, tempering (addition of oil to hardboard) or chemical modification of fibre before pressing into the final product. Generally speaking, a highly dimensionally stable composite product from lignocellulosic material made with conventional methods is not commercially viable, except for certain specialized and limited applications.
One such product is high density composite. This product can be distinguished from lower density composites by its appearance, in which the visibility of fibres or particles is virtually eliminated, as the product takes on a plastic like appearance and texture, and improved physical properties, demonstrating lower thickness swelling than their lower density counterparts. Generally speaking, this type of product is achieved by using very high quantities of synthetic resins (generally, in the range of 30–60%) mixed with lignocellulosic materials, which are then consolidated under heat and high pressure to produce a very dense, and tough, water resistant product, with a density of around 1300–1500 kg/m3. However, the costs of production are extremely high due to the high resin content, and the use of Kraft paper impregnated with phenolic resin as a feedstock, resulting in limited use of these products.
One method of producing a high density composite with a thickness of 7.0 mm involves the assembly of 44 layers of resin impregnated Kraft paper (consisting of 1 overlay paper impregnated with melamine resin, 1 decor paper impregnated with melamine resin, 41 Kraft papers impregnated with phenol resin and 1 balance paper impregnated with melamine or phenol resin). When consolidated under heat and pressure, the resulting product retains a uniform, plastic like appearance. However, the laminated material has the disadvantage of low dimensional stability under varying climatic conditions. In particular, the plate expands or shrinks significantly more in the transverse direction than the longitudinal direction as a result of the orientation of fibres in the Kraft paper feedstock.
An improvement of the paper layering method of producing high density composite product is taught in the U.S. Pat. No. 4,503,115, entitled “Plate shaped moulded article and process for its preparation and use.” In that patent, lignocellulose fibres are pressed together with thermosetting synthetic resins in the proportion 15 to 45% by weight to dry fibre, to a density between 900 to 1600 Kg/m3. The use of the fibre has the advantage of reducing costs, as the fibre is cheaper than Kraft paper, and improving the linear expansion properties, as the elimination of paper also eliminates the problem of differing dimensional variations resulting from the fibre orientation of the Kraft paper. However, the raw material costs of this method remain high, due to the high content of expensive resins, resulting in limited applications of the product.
A product that requires a high degree of dimensional stability, and particularly low levels of linear expansion, is laminate flooring. Laminate flooring is produced according to two general methods. The less expensive method of more recent development is the direct pressure lamination process (DPL) which involves pressing an abrasive resistant paper and melamine impregnated decorative paper on top of a fibre or particle coreboard with a melamine impregnated balance paper on the bottom of the coreboard. This laminate flooring is very popular. A second method to manufacture laminate flooring is by high pressure laminate (HPL). In the HPL process an abrasive resistant paper and melamine impregnated decorative paper and several sheets of phenol impregnated Kraft paper are assembled and pressed to a density of approximately 1,400 Kg/m3, for a relatively long time at a relatively high temperature. The resulting high pressure laminate is sanded on one side to improve its glue ability to the core board. The balance laminate is produced in a similar manner, being assembled with melamine and/or urea impregnated papers on the top and bottom with several sheets of phenol impregnated Kraft paper in the middle. The balance laminate is also sanded on one side to provide a suitable surface for gluing. The method to produce laminate flooring according to the HPL method is to assemble on the top a decorative HPL laminate (typically between 0.6–0.8 mm), in the middle, a core board of high density fiberboard or high density chipboard (density between 800 and 900 Kg/m3), and on the bottom, a balance laminate (typically between 0.6–0.8 mm). Laminate flooring by the HPL method is widely considered to be of better quality than the DPL method; however, because of the higher costs associated with the HPL method it is not as popular as the DPL method. According to a European consumer report, the best HDF core board has a thickness swelling of 7%.
In U.S. Pat. No. 5,017,319, EP 0,492,016, Canadian Patent 1,338,321 and EP 0,161,766, there is disclosed another process for making thermosetting resin adhesive and composite products from lignocellulosic material without the addition of synthetic resin. This process involves first using high pressure steam to decompose and hydrolyze the hemicellulose fraction, which accounts for 20–30% by weight of the lignocellulose, into low molecular weight water solubles. These water soluble materials are then utilized as a thermosetting adhesive to bond “in situ” the other components; i.e. the cellulose and lignin fractions of the lignocellulosic material under heat and pressure in a moulding operation to produce a reconstituted composite product. Since hemicellulose is one of the components of wood that is most hygroscopic and therefore most responsible for dimensional change in natural wood, its destruction renders the reconstituted product less hygroscopic and enhances dimensional stability. Furthermore, these patents also teach a secondary thermo-hydrolysis targeting and converting the cellulose fraction into water soluble resin material for the production of reconstituted composite products.
In addition, the low molecular weight water solubles derived from hemicellulose decomposition are able to permeate cell wall tissues and fill voids in the cellulose fibres, acting as a bulking agent. During the hot pressing operation, these water solubles polymerize, thermoset, and become water insoluble, thus eliminating or reducing water absorption. This bulking effect also enhances the dimensional stability of the reconstituted composite product. Thus, the water solubles, derived from hemicellulose decomposition, act as both a bonding and bulking agent to produce a moulded composite product with good mechanical strength and dimensional stability.
The lignin fraction, comprising 20–25% by weight of the lignocellulosic material, although decomposed and hydrolyzed by the high pressure steam into low molecular weight lignin and lignin decomposition products, which are water insoluble, is left in the hydrolyzed lignocellulosic material as a filler. The cellulose fibre (accounting for 45–50% of lignocellulose), which is not affected by the first steam treatment and which retains its physical integrity and is used as the backbone of the reconstituted composite product. The water soluble material functions as an adhesive, bonding “in situ” cellulose fibre and lignin together to yield a reconstituted composite product. The novelty of this process lies in the use of hemicellulose decomposition products as a binder, to improve physical properties of the composite products thus produced. The elimination of synthetic resin binders represents a significant breakthrough and improvement over conventional methods of manufacturing composite lignocellulosic products.